This invention relates to a method for preparing dehydrated and low moisture foods, such as fruits, vegetables and spices, quickly and with a minimum expenditure of energy, and to apparatus useful in carrying out this method. This invention further relates to a food dehydration method carried out at low temperatures, so that flavorful and aromatic volatiles are not driven from the product, and yet rapidly enough to prevent consequent spoilage of the food being processed.
Dehydration or drying is a method of food preservation in which water removal is used to control the growth of microorganisms and enzymatic activity that decompose foods in their natural state. Ideally, dehydrated foods are maintained in such a state that they can be reconstituted to approximately their original taste, if not shape, when contacted with sufficient water.
Dehydrated products are easier to handle than the natural products from which they are made, have far greater shelf lives, and are cheaper to ship and store. All of these advantages are particularly important in the case of institutional products such as military rations. Certain dehydrated products, such as onion and garlic powders, are sometimes preferred to their raw precursors, not only for their far greater shelf lives but also for their culinary versatility.
Many dehydrated products are hygroscopic in nature and will remain stable and unspoiled only if kept in a relatively low moisture atmosphere, as is provided, for example, by airtight packaging.
Most modern food dehydration processes are costly in terms of both energy and equipment. Evaporation using hot, dry air, usually produced by fossil fuel combustion, is the most common commercial dehydration method now in use. Hot air evaporation equipment is typically designed to be used for only one type of food product. Such equipment is often underutilized, and thus the cost of dehydration is further increased, especially when the material to be processed is only seasonally available. Dehydration processes other than hot air evaporation, such as freeze-drying, are also known, but require very sophisticated processing machinery which renders them uneconomical for all but a limited number of products.
The poor quality, flavor, aroma and texture of dehydrated products in comparison to the original materials is a further drawback of dehydration. Most of the blandness of dried foods results from the loss of volatile, aromatic, flavor-producing compounds during dehydration. Such losses occur because the amount of heat necessary to remove sufficient moisture also evaporates a substantial portion of the volatiles that give the natural product specific flavor and aroma characteristics. For example, the characteristic flavor and aroma of onions is due to the presence of components such as allyl propyl disulfide, pyruvates and a host of other unstable, volatile sulfur-containing compounds. The amount of pyruvates lost in onions during conventional dehydration may approach as much as 90 percent of the original amount. The adverse economic consequences of such flavor loss are obvious: one needs to use the conventionally dehydrated product in larger amounts than a product dehydrated by a process that minimizes the loss of flavor components to achieve an equivalent flavoring level. The more potent the flavor profile of the dehydrated product, the less one will have to use to achieve a desired result.
Flavor distortion during dehydration also results from air oxidation of labile flavoring components. This is a relatively slow process at low temperatures but the rate of oxidation increases rapidly on heating. Since isolation of the food being processed from the surrounding air is not generally feasible, shorter processing times and lower temperatures are usually required to minimize air oxidation.
Rehydration of dried foods to a condition resembling that of the parent material is often not possible, due to irreversible changes produced when animal or vegetable tissue is exposed to high temperatures. Free drying avoids many of the problems associated with processes utilizing heat, but prpoduces dried foods which tend to have spongy surfaces and is very expensive.
Most of the volatile matter driven from commercially dehydrated products during drying could be retained if processing took place as a lower temperature but, as indicated above, this has not been regarded heretofore as a practical solution because of the lengthy drying times required. The chances of fungal and bacterial contamination are increased as processing time is increased. Then two, certain undesirable oxidative and enzymatic chemical reactions that take place within the foods being processed can only be avoided if the material is treated more rapidly. For instance, when an onion is bruised, a slow enzymatic degradative activity is initiated. This degradation can be circumvented, however, if sufficient moisture is rapidly removed from the onion.
Numerous attempts have been made to overcome the disadvantages inherent in traditional dehydration techniques. Thus, for example, U.S. Pat. No. 4,383,376 teaches the use of various hydrophilic polymer substances which are used, in the form of contact dehydrating sheets placed against the surfaces of protein-containing foods in their natural state, to absorb water from the food. U.S. Pat. No. 3,732,627 discloses a desiccant and membrane combination used out of contact with the food being treated to remove liquids by drawing them off in the form of vapor. U.S. Pat. No. 3,386,838 discloses a process in which bananas are preserved by compressing them between juice-absorbent boards while exposing the boards to sunlight. Since excess fluids are not immediately removed, long processing times, normally 7-8 days, are necessary. Further, the amount of compression employed in this patent's method is so small that all the fluids being removed must either be absorbed or evaporated, and during this time, according to the patent, the fruit may have to be exposed to a sulfuric atmosphere in order to prevent mold formation.